Effect Of Ac Field Research Articles

AC and DC fields in optical lattices: quasienergy band structure

We study the effects of both AC and DC fields in the quasienergy band structure of cold atoms in optical lattices. The effects studied include Rabi oscillations between Bloch bands and dynamical band collapse in AC fields, which have already been observed experimentally. We also present some new results regarding the location of Wannier-Stark ladders in DC fields and the band structure of fractional Wannier-Stark ladders in DC/AC fields.

Electric field calibration in micro-electrode chambers by temperature measurements

To study the effect of strong ac-fields (up to 100 kV/m at frequencies between 0.25 and 40 MHz), suspended or adherently growing animal cells were cultivated on micro-electrode structures. The use of miniaturised electrodes has the advantage that cells can be exposed to electric fields in physiological solutions of high conductivity. A disadvantage is that the actual field strength near a cell depends both on its location and on other parameters. The problem is that stray dielectric losses in and near the chamber change the electric field in an undefined, frequency dependent manner. This paper compares three different techniques for estimating the field: (i) impedance measurements, (ii) the measurement of dielectrophoretic forces and (ii) a new calorimetric method within a test chamber. Advantages, limitations and possible applications are discussed.

Current density and ac field effects on 1/f noise in spin valve sensors

Low frequency noise in patterned NiFe/Co/Cu/Co/FeMn (ΔR/R>9%) spin valves was investigated. The spin valves were sputter deposited and patterned by lift-off to form 75 μm×1000 μm lines. Noise power spectra were measured in the frequency range from 1 Hz to 10 kHz for ac and dc fields applied both parallel and perpendicular to the easy axis. An increase in the low frequency noise power spectrum was observed for an ac field applied along the easy axis. Increasing the ac drive frequency from 500 Hz to 5 kHz reduced the low frequency noise power to near the zero field background level.

The ac and dc electric field effects on photoreceptor rods of Xenopus laevis

In this paper we investigate the response of photoreceptor rods from frog retina to treatment by 1 MHz ac fields followed by dc pulses. Photoreceptor rods brought in close contact by dielectrophoresis (1 MHz field frequency and 0.5-kV/cm threshold intensity) were fused or intermingled by application of dc pulses. During application of dc pulses some rod cells were damaged, releasing discs into the suspension. Discs dielectrophoresis led then to their association in pairs or clusters, which could be fused by higher pulses, resulting in vesicles of different sizes and shape patterns. Rods rotation around their long axis as well as discs elongation were observed and monitored at different field intensities. These field effects allow new insights into photoreceptor rods morphology as well as into membrane dynamics during discs formation and decay.

A comparative study of the ac-field dependence of ‘soft’ and relaxor PLZT ferroelectrics

Significant differences in the ac-field dependence of the dielectric constant, polarization hysteresis response, and the fatigue behavior have been found between low La-content “soft” and high La-content relaxor ferroelectric behaviors in lead zirconate titanate (PZT) ceramics. X-ray diffraction and transmission electron microscopy studies revealed a decrease in domain regularity with La concentration in “soft” ferroelectrics. This domain breakdown is believed to be responsible for the anomalous ac field effects. An ac field induced relaxor behavior observed in low La content “soft” PZT requires an extended concept of relaxor behavior.

Comparison of DC and AC field effects on dielectric properties of lead magnesium niobate relaxor: Study of single crystals and ceramics

Dielectric permittivity of PbMg1/3Nb2/3O3 (PMN) relaxor was investigated as a function of DC bias and AC driving field. In single crystals, DC and AC field effects exhibited very different behavior, including anisotropy and sign of the effect. It was shown that the model of PMN, in which the dielectric response of the material to the AC field was controlled by a side-way motion of the interphase boundaries of the polar regions, provided a good qualitative interpretation of the observed phenomena. Comparison of the DC and AC field effects measured in ceramics and single crystals demonstrated that the data obtained for ceramics correspond to the averaging of the non-linear effects over random orientations of the grains.

Effects of low-intensity AC and/or DC electromagnetic fields on cell attachment and induction of apoptosis.

Rat tendon fibroblast (RTF) and rat bone marrow (RBM) osteoprogenitor cells were cultured and exposed to AC and/or DC magnetic fields in a triaxial Helmholtz coil in an incubator for up to 13 days. The AC fields were at 60 and 1000 Hz and up to 0.25 mT peak to peak, and the DC fields were up to 0.25 mT. At various combinations of field strengths and frequencies, AC and/or DC fields resulted in extensive detachment of preattached cells and prevented the normal attachment of cells not previously attached to substrates. In addition, the fields resulted in altered cell morphologies. When RTF and RBM cells were removed from the fields after several days of exposure, they partially reattached and assumed more normal morphologies. An additional set of experiments described in the Appendix corroborates these findings and also shows that low-frequency EMF also initiates apoptosis, i.e., programmed cell death, at the onset of cell detachment. Taken together, these results suggest that the electromagnetic fields result in significant alterations in cell metabolism and cytoskeleton structure. Further work is required to determine the relative effect of the electric and magnetic fields on these phenomena. The research has implications for understanding the role of fields in affecting bone healing in fracture nonunions, in cell detachment in cancer metastasis, and in the effect of EMF on organisms generally.

Mechanical Vibrations of Smectic Cells Under Fast Field Reversal

Abstract It is known that ferroelectric liquid crystal cells vibrate mechanically under the effect of ac fields. The vibrations are mainly due to the backflow and electroclinic effects. For small director oscillations the effect is mainly linear and goes with the frequency of the field. For larger oscillations higher harmonics appear too. We studied the practically interesting case when, due to rectangular excitation, the director switches fully between two stable positions. In the experiments bookshelf textures of (R)-MHPOBC and FLC6430 (from Hoffmann La Roche) were used. The accelerations of the cover plates due to the reversal of the field were detected in three orthogonal directions. The switches cause strong vibrations of the plates (lasting less than a millisecond,. acceleration ∼10m/s2). The vibration is largest at the SmCα*-Sm A phase transition indicating that the dominating effect is electroclinic in origin.

Shear stress and aggregation kinetics of electrorheological fluids under alternating electric fields

The effect of periodic ac and pulse electric fields on the shear stress of an electrorheological fluid is investigated by computer simulations. It is observed that as the frequency of the applied field (ω) is increased under a constant rate of shear (γ), the shear stress passes through a minimum. It is found that there is a distinct difference in the aggregation processes due to the induced electric dipoles at frequencies above and below this critical frequency; for low frequencies the clustering follows the electric field variation, while at high frequencies the clusters are unable to completely break up. These considerations of the clustering process lead us to propose a scaling law for the shear stress: σxy=σxy(ω/γ). This result agrees with experiments and with computer simulations.The effect of periodic ac and pulse electric fields on the shear stress of an electrorheological fluid is investigated by computer simulations. It is observed that as the frequency of the applied field (ω) is increased under a constant rate of shear (γ), the shear stress passes through a minimum. It is found that there is a distinct difference in the aggregation processes due to the induced electric dipoles at frequencies above and below this critical frequency; for low frequencies the clustering follows the electric field variation, while at high frequencies the clusters are unable to completely break up. These considerations of the clustering process lead us to propose a scaling law for the shear stress: σxy=σxy(ω/γ). This result agrees with experiments and with computer simulations.

Nonlinearity in the I-V characteristics in thin lipid films: Effect of AC and DC fields

The current-voltage characteristics of bilayer lipid membranes of oxidized cholesterol separating two bathing solutions have already been extensively studied under a DC electric field. The observed deviation from linearity at high field has been explained by field-induced pore formation, which then act as ion channels in the membrane. Using thin films of oxidized cholesterol and of dipalmitoyl phosphatidylcholine, we have reported for the first time similar deviation from nonlinearity in the DC I-V characteristics when the applied field is above 40 V/cm. Upon application of an AC field, the conductivity increases as square of frequency, while the nonlinear nature of the I-V characteristic curve is still retained at all frequencies up to 5,000 Hz. Our results indicate that besides pore formation, the intrinsic electrical properties of the constituent lipid molecules are also responsible for the observed nonlinearity.

Effect of ac field on the limiting blow-off flow-rate value for a diffusion propane-hydrogen flame

Effect of ac field on the limiting blow-off flow-rate value for a diffusion propane-hydrogen flame

Dielectrophoretic measurement of bacterial motor characteristics

Two methods of measuring bacterial motor characteristics using AC field effects in a microfabricated electrode system are presented. One is the measurement of the external force-velocity characteristics (F-v) of swimming bacteria. Electrostatic orientation of bacteria parallel to the field lines is used to guide the bacterial locomotion along a line. Dielectrophoresis is used to apply an external force, either forward or backward, to the swimming bacteria, and the velocity of locomotion is measured to obtain the F-v curve. The other approach is to measure torque-speed characteristics (T-w) of the motor. Electrorotation is used to apply external torque to the tethered cells, and by changing the applied torque and measuring the rotation speed, the T- omega curve is obtained. The results show that the motor generates approximately constant torque in the measured range of omega (0-100 Hz), regardless of the direction of rotation. >

Experimental search for combined AC and DC magnetic field effects on ion channels

The hypothesis that specific combinations of DC and low frequency AC magnetic fields at so-called cyclotron-resonance conditions could affect the transport of ions through ion channels, or alter the kinetics of ion channels (opening and closing rates), has been tested. As a model system, the ion channels formed by gramicidin A incorporated in lipid bilayer membranes were studied. No significant changes in channel conductance, average lifetime, or formation rate as a function of applied fields could be detected over a wide range of frequencies and field strengths. Experiments were carried out to measure the time-resolved single-channel events and the average conductances of many-channel events in the presence of K+ and H+ ions. The channel blocking effect of Ca++ was also studied.

Shear Stress and Aggregation Kinetics of Electrorheological Fluids under Alternating Electric Fields

The effect of periodic ac and pulse electric fields on the shear stress of an electrorheological fluid is investigated by computer simulations. It is observed that as the frequency of the applied field (ω) is increased under a constant rate of shear (γ), the shear stress passes through a minimum. It is found that there is a distinct difference in the aggregation processes due to the induced electric dipoles at frequencies above and below this critical frequency; for low frequencies the clustering follows the electric field variation, while at high frequencies the clusters are unable to completely break up. These considerations of the clustering process lead us to propose a scaling law for the shear stress: σxy=σxy(ω/γ). This result agrees with experiments and with computer simulations.The effect of periodic ac and pulse electric fields on the shear stress of an electrorheological fluid is investigated by computer simulations. It is observed that as the frequency of the applied field (ω) is increased under a constant rate of shear (γ), the shear stress passes through a minimum. It is found that there is a distinct difference in the aggregation processes due to the induced electric dipoles at frequencies above and below this critical frequency; for low frequencies the clustering follows the electric field variation, while at high frequencies the clusters are unable to completely break up. These considerations of the clustering process lead us to propose a scaling law for the shear stress: σxy=σxy(ω/γ). This result agrees with experiments and with computer simulations.

Nature of the Dielectric Anomaly at 161 K in Thiourea

The low frequency dielectric constant of thiourea exhibits a sharp peak reaching 10 4 ∼10 5 at 161.6 K ( T x ) in the ferroelectric phase. From the study of the effects of dc and ac fields on this anomaly, we infer that the anomaly is produced by domain wall motion. While the static dielectric constant tends to diverge toward T x , the dielectric relaxation time never shows a slowing down. This fact and the result of switching experiments suggest strongly that the anomalous increase in the dielectric constant is caused not by the softening in the restoring force acting on domain walls, but by the anomalous increase in the domain wall density. A tentative model is proposed which is capable of explaining this anomalous phenomenon.

Effects of AC field on streaming electrification of solution of a surfactant in insulating liquid

Effects of AC field on streaming electrification of solution of a surfactant in insulating liquid

The effect of nonsteady electric fields on sooting flames

The effects of transient dc and 50-Hz ac fields acting on a sooting acetylene diffusion flame were compared with those of steady dc fields. The object was to separate the velocities of charged soot particles from those of the ionic wind by making use of the difference in their relative evolution times. This has not been considered previously, largely beacuse much of the research was carried out on flat counterflow flames, but is here shown to be important for conventional flame shapes. Laser Doppler and scattering methods were used to determine particle velocities and sizes. Soot particles of the order of 10 μm diameter were found to have mobilities of the order of 5 × 10 −6 m 2 s −1 V −1 and zero for particles exposed to a flux of positive and negative charge carriers, respectively. Major perturbations to flow due to ionic wind effects set in after a delay of the order of 10 ms. Similar results were obtained whether the transients followed the switching on of a dc field or occurred during the 50-Hz cycling of the ac field.

Effect of ac field on the phase conjugate signal in BSO

This paper presents results on the dependence of a phase conjugate signal in Bi 12SiO 20 (BSO) crystal on the frequency of an applied sinusoidal ac electric field. Large enhancement in the phase conjugate signal is observed for the ac field compared to that produced by the equivalent dc field. The conjugate signal shows a strong dependence on the ac field frequency. In addition, the variation of the diffraction grating signal with the ac field frequency is studied.

Experiments on the Interaction of Electromagnetic Fields with Mammalian Systems.

The results of a series of measurements on the interaction of electromagnetic fields with cell ensembles in vitro are described. The measurements include the effect of fields on population doubling time, 3H-TdR uptake, membrane transport, and galvanotropism. No measureable effect of AC fields was found, and in particular no evidence was found for a cyclotron-resonant mechanism in the ion transport across cell membranes.

界面活性剤を含む絶縁液体の交流電圧印加時の流動帯電

界面活性剤を含む絶縁液体の交流電圧印加時の流動帯電